The present invention relates to electroless gold plating compositions. More particularly, the present invention is directed to autocatalytic gold plating compositions used for plating gold onto gold.
Electroless plating is useful in applications requiring coatings for complex shapes This capacity makes electroless plating techniques particularly useful in the electronics industry, for example, in the metallization of conductors and insulators in printed circuit boards. Base metals such as nickel and copper are often used in electroless plating processes to metallize conductors and insulators. However, because of its low contact resistance and beneficial effects on bonding leads, electroless gold plating is continually desired for coating complex shapes and electrically isolated tracks and bonding pads in printed circuit boards. It has been found, however, that electroless gold plating baths are not as easy to operate as the electroless base metal plating baths, for example, with respect to bath stability and plating rate.
The term "autocatalytic" is used herein to describe a plating system which is capable of depositing gold on a gold substrate. Autocatalytic gold plating is advantageous, for example, for increasing the thickness of existing gold surfaces which are too thin for some uses. Autocatalytic electroless gold plating compositions are known in the art. Reference may be made, for example, to U.S. Pat. Nos. 3,700,469; 3,917,885; and 4,337,091.
U.S. Pat. No. 3,700,469 to Okinaka discloses an autocatalytic electroless gold plating bath containing a soluble gold cyanide complex ion, excess free cyanide to stabilize the gold cyanide complex ion, an alkaline agent as a pH adjustor, and an alkali metal borohydride or dimethylamine borane as a reducing agent. Although a truly autocatalytic plating bath, the Okinaka bath has several limitations including instability, low plating rate (about 1 micron per hour), difficulty of bath replenishment, and sensitivity to nickel ions in solution.
U.S. Pat. No. 3,917,885 to Baker discloses an autocatalytic electroless plating bath purportedly having improved stability by adding an alkali metal imide complex of the metal to be plated, e.g., gold. However, the Baker bath has been found to have the same problems as the Okinaka bath, especially sensitivity to nickel contamination and deteriorating plating rate.
U.S. Pat. No. 4,337,091 to El-Shazly et al. discloses the use of trivalent gold metal complexes as the source of gold in an electroless gold plating bath, the reducing agent being any of the borohydrides, cyanoborohydrides or amine boranes that are soluble and stable in aqueous solution. A later version of the El-Shazly bath, disclosed in U.K. Patent Application G.B. No. 2/2/444A, uses a mixture of trivalent and monovalent water-soluble gold cyanide complexes. The El-Shazly baths suffer from the same limitations as the prior art baths described above.
In autocatalytic plating, oxidation/reduction reactions begin simultaneously when the substrate is immersed in the plating bath. These reactions occur at the surface of the metal or metallized substrate. At the substrate, the gold ions accept electrons from the reducing agent and deposit a gold film on the substrate. Initially, the reducing agent reacts on the substrate, giving electrons to the metal ions and being converted to its oxidized form. The gold film then catalyzes the reaction and causes it to continue autocatalytically.
In an electroless bath containing an alkali metal gold cyanide complex, an alkali metal cyanide complexing agent, an alkali metal hydroxide, and dimethylamine borane as a reducing agent, cyanide is a strong poison for the oxidation of the reducing agent. However, cyanide is necessary to prevent the spontaneous decomposition of the bath by reaction [1] below: EQU (CH.sub.3).sub.2 NHBH.sub.3 +4OH.sup.- +3Au(CN).sub.2.sup.-= EQU [1] (CH.sub.3).sub.2 NH+BO.sub.2.sup.- +1.5 H.sub.2 +2H.sub.2 O+3Au+6CN.sup.-
The oxidation kinetics and plating rate of the bath may be increased by decreasing cyanide or increasing the reducing agent and the alkali metal hydroxide. In doing so, however, the bath becomes more prone to spontaneous decomposition by reaction [1]. Another way in which the oxidation kinetics and plating rate may be increased is by adding a substance to the bath which would promote oxidation of the reducing agent without entering into reaction [1].
The present invention is based on the discovery that carbonate acts as such a promoter.
In an electroless bath containing an alkali metal hydroxide, alkali metal carbonate will be continuously formed in the bath due to reaction of the hydroxide with carbon dioxide in the air. It has been found, however, that the addition of carbonate initially to a plating bath having a pH within the range of about 10 to about 14 and comprising a water soluble alkali metal monovalent gold cyanide complex and a water-soluble alkali metal cyanide as a complexing agent results in an autocatalytic electroless plating bath having improved stability and a higher plating rate as compared to a bath to which carbonate was not initially added.
The use of carbonate in electroless plating baths is known in the art. Reference may be made, for example, to U.S. Pat. Nos. 3,515,571 and 4,091,128, to Levy and Franz, respectively. These references describe non-autocatalytic electroless gold plating baths containing gold salt, a complexing agent for the gold salt, and hydrazine as the reducing agent. In both references, carbonate is used as the complexing agent for gold. Neither reference discloses the presence of cyanide in the baths. In the plating bath of the present invention, cyanide is present as the complexing agent for gold, thereby allowing the carbonate to be free to accelerate the oxidation of the reducing agent. Carbonate will not function as an accelerator in the present invention if it is complexed with the gold.
The use of carbonate as an accelerator for an alkali metal borohydride is described in U.S. Pat. No. 4,092,154 to Dietz, Jr. et al. The Dietz patent discloses a precipitating agent for recovering precious metals such as gold from aqueous cyanide solutions, wherein the precipitating agent contains aluminum powder and a reducing agent which can be an alkali metal borohydride. The precipitating agent can also contain an alkali metal carbonate as an activator for the reducing agent. According to column 3, lines 59 et seq, of the patent, the addition of the alkali metal carbonate to the precipitating agent increased the speed and completeness of the precipitation reaction.
Unlike the situation in Dietz, Jr. et al., the precipitation of gold from the plating solution in the present invention is undesirable and is avoided despite the action of the carbonate as an accelerator for the reducing agent. Instead, in the present invention, carbonate in combination with the other components of the bath provide a plating bath having excellent stability against precipitation of the gold from solution and improved rate in plating the gold onto a gold substrate immersed in the solution.